This invention relates to the synthesis of hydrocarbons from a methane source. A particular application of this invention is a method for converting natural gas to more readily transportable material.
A major source of methane is natural gas. Other sources of methane have been considered for fuel supply, e.g., the methane present in coal deposits or formed during mining operations. Relatively small amounts of methane are also produced in various petroleum processes.
The composition of natural gas at the wellhead varies but the major hydrocarbon present is methane. For example, the methane content of natural gas may vary within the range from about 40 to about 95 volume percent. Other constituents of natural gas include ethane, propane, butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and nitrogen.
Natural gas is classified as dry or wet depending upon the amount of condensable hydrocarbons contained in it. Condensable hydrocarbons generally comprise C.sub.3 + hydrocarbons carbons although some ethane may be included. Gas conditioning is required to alter the composition of wellhead gas, processing facilities usually being located in or near the production fields. Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane.
Large scale use of natural gas often requires a sophisticated and extensive pipeline system. Liquefaction has also been employed as a transportation means, but processes for liquefying, transporting, and revaporizing natural gas are complex, energy-intensive and require extensive safety precautions. Transport of natural gas has been a continuing problem in the exploitation of natural gas resources. It would be extremely valuable to be able to convert methane (e.g., natural gas) to more readily handleable or transportable products. Moreover, direct conversion of olefins such as ethylene or propylene would be extremely valuable to the chemical industry.
Recently, it has been discovered that methane may be converted to higher hydrocarbons (e.g., ethane, ethylene and higher homologs) by contacting methane with a reducible metal oxide as a selective oxygen source. As the methane is converted to hydrocarbon products and coproduct water, the active oxygen of the metal oxide is depleted, resulting in a reduced metal oxide. The reduced metal oxide is relatively inactive for the oxidative conversion of methane but active oxygen may be replaced by regenerating of a reducible metal oxide. Such regeneration is accomplished by reoxidation of the reduced metal oxide.
Reducible oxides of several metals have been identified which are capable of converting methane to higher hydrocarbons. Oxides of manganese, tin, indium, germanium, lead, antimony and bismuth are particularly useful. See commonly-assigned U.S. patent application Ser. Nos. 522,925 (now U.S. Pat. No. 4,443,649); 522,944 (now U.S. Pat. No. 4,444,984); 522,942 (now U.S. Pat. No. 4,443,648); 522,905 (now U.S. Pat. No. 4,443,645); 522,877 (now U.S. Pat. No. 4,443,647); 522,876 (now U.S. Pat. No. 4,443,644); and 522,906 (now U.S. Pat. No. 4,443,646), all filed Aug. 12, 1983, the entire contents of each being incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 522,935, filed Aug. 12, 1983, discloses and claims a process which comprises contacting methane with an oxidative synthesizing agent under elevated pressure (e.g., 2-100 atmospheres) to produce greater amounts of C.sub.3 + hydrocarbon products.
Commonly-assigned U.S. patent application Ser. No. 522,938, filed Aug. 12, 1983, discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with particles comprising an oxidative synthesizing agent which particles continuously recirculate between two physically separate zones--a methane contact zone and an oxygen contact zone.
In a typical application of the foregoing processes for the oxidative conversion of methane, methane feed is contacted with a reducible metal oxide and regeneration is accomplished separately by contacting the reduced metal oxide with an oxygen-containing gas (e.g., air). Thus, a cyclic redox process results in which methane reaction and reoxidation of the metal oxide "reagent" are performed separately and repeatedly for a continuous process.
Such a procedure presents several disadvantages for large scale continuous operation. One disadvantage is the large quantity of solid cycling between methane reaction and reoxidation in such a way that the methane and oxygen are not mixed. Another disadvantage is the necessity of developing a composition that is resistant to mechanical attrition and repeated exposure to reductive and oxidative environments.
Hinsen and Baerns recently reported studies of a continuous mode for the oxidative coupling of methane wherein regenerating air is cofed with the methane feed. Hinsen, W. and Bearns, M., "Oxidative Kopplung von Methan zu C.sub.2 -Kohlenwasserstoffen in Gegenwart unterschiedlicher Katalysatoren", Chemiker-Zeitung, Vol. 107, No. 718, pp. 223-226 (1983). Using a catalyst based on lead oxide and gamma-alumina in a fixed bed reactor operated at 1 atmosphere total pressure and 600.degree.-750.degree. C., they report results of approximately 53% selectivity to ethane and ethylene at 8% methane conversion for a feed consisting of about 50% methane, 25% air and 25% nitrogen. Other metal oxides studied by Hinsen and Baerns included oxides of Bi, Sb, Sn and Mn.